1. Field of the Invention
The present invention relates to a surface acoustic wave device that is used as a resonator or a bandpass filter and to a duplexer including a transmission/reception filter that includes a surface acoustic wave device.
2. Description of the Related Art
To date, surface acoustic wave devices have been widely used as resonators and bandpass filters. Such surface acoustic wave devices typically include a comb-shaped interdigital transducer (IDT) electrode formed on the surface of a substrate having a piezoelectric property.
The operation frequency of surface acoustic wave devices has been shifting toward higher frequencies from a band of several hundred MHz to a band of several GHz in conjunction with the shift toward higher frequencies and the continuing size reduction of mobile communication devices, and together with this, high-frequency devices such as bandpass filters that use surface acoustic wave devices have been becoming progressively smaller in size. The pattern width of an IDT electrode needs to be very small in order to realize a high frequency and an IDT electrode needs to be formed with a pattern width of around 0.50 μm in a high-frequency bandpass filter having a center frequency of 2 GHz, for example.
As disclosed in Japanese Unexamined Patent Application Publication No. 2002-26685, an IDT electrode experiences a strong stress from a surface acoustic wave propagating along the surface of a piezoelectric substrate when a high-level voltage is applied to an IDT electrode having a very small pattern width as described above. If this stress exceeds the critical stress of the electrode film, stress-induced electrochemical migration occurs. In the case where Al is used as the material of the IDT electrode, Al atoms move along the crystalline grain boundaries due to the stress-induced electrochemical migration, and consequently, hillocks and voids are generated in the IDT electrode. Therefore, the IDT electrode is damaged and a point is reached where the characteristics of the surface acoustic wave device are degraded such that increases in electrical short circuits and insertion loss, and lowering of the Q value of a resonator occur.
In order to solve the above-described problem, a proposal has been made to use Cu as the material of the IDT electrode. As disclosed in Japanese Unexamined Patent Application Publication Nos. 2002-26685 and 2007-235711, there is a problem regarding adhesion between an IDT electrode formed of Cu and a piezoelectric substrate. Therefore, an adhesive layer is provided between a main electrode layer and the piezoelectric substrate, and as a result, the adhesion and electric power handling capability of the IDT electrode can be improved. For example, one metal material selected from a group consisting of Ti, Ni, Cr and Ni—Cr is used as the material of the adhesive layer.
However, it has been recognized that, even in the case of a surface acoustic wave device in which an adhesive layer is provided between the main electrode layer and the piezoelectric substrate, the insertion loss of a bandpass filter increases when a high-level voltage is applied for a long time in a high-temperature environment to a bandpass filter in which such a surface acoustic wave device is used.
Consequently, adding a surface acoustic wave device to a terminal section of the bandpass filter, adding a direct-current cutting capacitor outside the bandpass filter and so forth in order to reduce the voltage applied to the bandpass filter have been considered, but this would cause the size of the product to be increased in such cases.
It is thought that the increase in the insertion loss of the bandpass filter described above is caused by the metal that forms the main electrode layer of the IDT electrode diffusing into the piezoelectric substrate. The metal that forms the main electrode layer fuses with the crystal that forms the piezoelectric substrate and causes electrical short circuiting of the IDT electrode to occur. Such an increase in insertion loss cannot be sufficiently suppressed even when an adhesive layer formed of a material that does not readily diffuse into the piezoelectric substrate is provided between the main electrode layer and the piezoelectric substrate.